Covering seeds

3. Once all seeds have been placed/planted, sprinkle coarse grade vermiculite or additional potting soil over the seeds and soil surface to a thickness just sufficient to cover both seeds and soil. Using the watering pail with a diffusing/sprinkler head, lightly add water to thoroughly wet the vermiculite.

Vermiculite is available at most horticultural supply stores or web sites. We use Sunshine coarse grade (Sun Gro Horticulture, Bellvue, WA), but largely out of habit. Alternative brands should be equally suitable. Vermiculite is a mined and processed mineral similar in terms of multilayered structure to mica. Upon heating, microdroplets of water trapped between the layers turn to steam, causing the mineral to expand as the layers separate slightly. When used for growing our plants, vermiculite serves two purposes: first, upon wetting, water is drawn between layers of the mineral and held in place by capillary action. With the wet soil beneath and the wet vermiculite around and above, the seeds are placed in a very moist—but not suffocating—environment that promotes rapid germination. Second, by shielding the organic material in the soil from light, the vermiculite layer limits or prevents growth of algae during the plant life cycle.

Vermiculite Vermiculite covered tray

Figure 5: Magnified (6x) view of highly-layered vermiculite (left) and a covered flat after planting seeds (right). Gently water the flat from above to moisten the vermiculite. If vermiculite is unavailable, a light layer of potting soil may be used to cover seeds.